1 98 THE HAEMOFLAGELLATES 



warm-blooded Vertebrates for which the transmitting agent is 

 known, this is an insect, generally a member of the Diptera ; in 

 that of Trypanosomes of cold-blooded Vertebrates the same role is 

 usually played by an Ichthyobdellid leech (Piscine forms), but 

 possibly now and again by an Ixodes (some Amphibian or Eeptilian 

 forms). 



The actual relation between the parasite and the transmitting 

 Invertebrate has long been questioned, and there are still some 

 very important instances in which the real state of affairs is not 

 certain. But it would seem, from the. results of recent work, 

 that in most cases some Invertebrate or other acts as a true 

 alternate host. Thus, so far as leeches are concerned in connec- 

 tion with the Trypanosomes of fishes, the investigations of Leger 

 (50), Brumpt (10-12), and Keysselitz (27) have made it clear that 

 the parasites not only live quite normally, but undergo a definite 

 evolution in particular organs of leeches which have fed on infected 

 fish. Frequently this further development can only proceed, at 

 least to its full extent, in a certain leech to the exclusion of others 

 (e.g. in a Hemiclepsis and not in a Piscicola, or vice versa) ; this restric- 

 tion points distinctly to the leech in question being a specific natural 

 host. Again, according to the celebrated researches of Schaudinn 

 (75) on an Avian Trypanosome, Trypanomarpka (Trypanosoma) 

 noctuae, a species of gnat (Culex) provides the alternate host, 

 in which a complex part of the life -cycle takes place. It is 

 interesting to note that, as might be expected, there is a regular 

 periodicity in the infectivity of the gnat ; that is, it can only 

 transmit the infection after such and such an interval has elapsed 

 since the meal when it became itself infected. Coming, lastly, to 

 the Mammalian forms, Prowazek (68) has described phases of 

 development of T. lewisi in the rat-louse (Haematopinus sp.), 

 and considers that this insect serves as a true Invertebrate host ; 

 though he was not able to prove the actual transmission of the 

 parasites back to the rat by means of it. 1 



Interest and discussion has mostly centred, however, upon the 

 part played by the transmitters of the lethal Trypanosomes, and 

 it is only quite recently that any light can be said to have been 

 thrown upon the subject. 



It has for some time been generally recognised that, in many cases at 

 any rate, a particular biting-fly is chiefly responsible for the spread of a 

 particular parasite in an infective district. In such cases, a striking 

 coincidence usually exists between the area over which a certain trypanoso- 

 mosis is prevalent and the zone of distribution of a certain fly. Thus, of 

 two well-known African Trypanosomes, one, T. briicii, the cause of Nagana 



1 This has been effected, however, by earlier observers (Rabinowitsch and 

 Kempner) by means of fleas, which are possibly the " right " insects. 



